1. Field of the Invention
The present invention relates generally to vehicle brake drums, and more particularly to devices and systems for measuring the inside or brake surface diameter of brake drums, and more particularly still to devices and systems for inspecting and measuring the inside or brake surface diameter of a brake drum without requiring removal of the wheel and brake drum from a vehicle wheel axle.
2. Preliminary Discussion and Prior Art
Brake drums are available in various sizes and styles, and are selected for use according to their specifications and depending upon the size and type of vehicle with which a drum is intended to be used, and in particular depending upon the braking force required to stop such vehicle. The required braking force is essentially divided between the wheels of the vehicle and is directly related to the weight of the vehicle and the speed at which the vehicle is travelling. To stop a vehicle travelling at a given speed, if the weight of the vehicle is doubled, the braking force required to stop the vehicle is also doubled, while if the speed is doubled, the braking force required to stop the vehicle is quadrupled,
When the brakes are applied in an air brake or air actuated hydraulic brake system including brake drums, the brake shoe linings mounted inside the brake drum are pressed outwardly into contact with the inside surface of the revolving brake drums with a force great enough, also considering the traction of the wheels with the road surface, to create adequate friction to stop the vehicle wheels from turning. The friction between the brake shoe linings and drum generates large amounts of heat as the mechanical energy of the revolving brake drums and wheels is converted into heat energy. Such heat is absorbed by the metal brake drums and then dissipated into the atmosphere.
The amount of heat a brake drum can absorb is largely dependent upon the thickness of the metal. Over time, as the brakes are repeatedly applied, the brake drum braking surfaces are slowly worn. The force of the brake pad linings against the inside surface of a brake drum causes wear on such surface, and in turn the thickness of the drum wall is decreased, reducing the ability of the drum to absorb and dissipate heat. If the metal drum is worn too thin, excessive heat buildup can cause drum distortion and weakening. All brake drums are accorded a maximum wear allowance, or maximum inside diameter, beyond which the drum should be replaced because a vehicle having such drum installed cannot be safely operated. If the maximum inside diameter of a brake drum is exceeded, the braking force generated by the brakes can be severely compromised. A reduction in braking power, or fading, can occur when during braking the heated brake drums expand away from the brake linings. There is also an unacceptable risk of excessive heat buildup causing brake failure, and the brake lining material will lose its effectiveness with overheating. One or more drums on a vehicle can also wear more quickly if all of the brakes are not properly balanced or adjusted, since the other wheels must do more than their share of the work required to brake the vehicle, and possibly become damaged by an excessive amount of heat.
Conventionally, inspecting the inside diameter of a brake drum to determine if it is at or near its safe maximum operation diameter, above which it must be replaced, or if worn or scored whether or not the brake surface can be machined for further use, has required that both the wheel and drum be removed from the vehicle, after which a measuring tool such as a gauge or caliper is used to measure the inside diameter. See for example U.S. Pat. No. 2,791,032 issued to H. B. Barrett on May 7, 1957, entitled “Brake Drum Gauge”; U.S. Pat. No. 4,520,568 issued to D. A. Drenner on Jun. 4, 1985, entitled “Brake Drum Gauge”; U.S. Pat. No. 6,957,498 issued to E. J. Tsai on Oct. 25, 2005, entitled “Brake Drum Caliper”; U.S. Published Patent Application 2008/0189971 filed by Lin Wo entitled “Electronic Gauge Disc and Drum Brake Gauge” and published on Aug. 14, 2008; and U.S. Published Patent Application 2009/0223075 filed by Lin Wo entitled “Gage for Measuring Drum Brake Inside Diameter”.
A significant drawback of these prior art brake drum inspection tools is that the process of removing each wheel and brake drum from a vehicle, inspecting the drum, and then remounting each wheel and either a new drum or the existing drum, is extremely time consuming, particularly where the vehicle is a tractor trailer or other large vehicle. Full vehicle inspections are therefore expensive both in terms of the amount of manpower and time spent making such inspections, but also in terms of down time for the vehicle, which is essentially out of service until the inspection is completed and either the existing or replacement drums and wheels are remounted on the vehicle. The long time required to complete a full inspection also limits the number of inspections that can be performed, which in turn leads to an increased number of potentially unsafe vehicles, including tractor trailers but also transit vehicles such as buses, motorcoaches, and the like, on the roads. In fact, a large number of highway accidents are known to be caused by faulty brake systems, often at least in part due to overly worn brake drums wherein the maximum inside diameter has been exceeded.
A few calipers for measuring the inside diameter of a brake drum without first requiring removal of the wheel and drum are available in the prior art, of which the following can be particularly mentioned.
U.S. Pat. No. 4,782,595 issued to A. Diewert on Nov. 8, 1988 entitled “Brake Drum Measuring Apparatus” discloses a measuring device comprised of a receiving bracket and gauge member having a pair of downwardly depending arms with calibration pins on their ends. A bar for properly calibrating the gauge is also provided. The arms of the Diewert device are U-shaped, which allows the ends of the arms and calibration pins to be inserted in the brake drum through inspection plates or other openings in the drum cover while mounted on a vehicle.
U.S. Pat. No. 6,216,357 issued to R. Victor et al. on Apr. 17, 2001 entitled “Wheels-On Brake Drum Gauge” discloses another brake drum gauge for measuring the inside diameter of a brake drum while the wheel is mounted on a vehicle comprised of an elongated bar with a pair of V-shaped arms one of which is fixed and the other which is movable. The V-shaped arms include an upright portion parallel to the reference plane of the bar, and an angled portion having a fixed anvil with a tip thereon. Means for measuring the distance between the anvil tips when placed on diametrically opposite sides of the drum is provided. As with Diewert, the V-shape avoids interference with the vehicle axle as would occur with a conventional straight caliper tool.
U.S. Pat. No. 7,040,151 issued to M. R. Graham et al. on May 9, 2006 entitled “Brake Evaluation Gauge” discloses another manual gauge for measuring the inside diameter of a brake drum while mounted on a vehicle comprised essentially of a shank having several toes and recesses spaced apart a predetermined distance along the length of the shank, whereby the recess is inserted over a ridge on the brake drum, with the toe of the first end of the gauge contacting the drum. If the ridge contacts the interior surface, this indicates that the brake drum wear exceeds a maximum allowable wear, but if it doesn't the drum wear does not exceed the maximum allowable wear.
While the above prior art devices may be suitable for their intended use, each has one or more shortcomings which it is believed makes them unacceptable for widespread use by vehicle inspectors. When mounted on a vehicle axle, the brake drum is of course positioned behind or underneath the wheel, and so the only way to access the inside surface of the brake drum is through an opening facing inwardly on the side opposite the wheel, which is inconvenient and awkward to access. Thus, using any of the prior art devices to perform an inspection with the brake drum still mounted on the vehicle, particularly along a roadside or the like as opposed to use in a specially designed mechanic's garage, is in practice quite difficult. For example, in order to use either the Diewert or Victor et al. gauges, an inspector must crawl almost completely under the vehicle and then properly position the gauge in the brake drum to obtain an accurate reading, which will very likely also require use of an auxiliary light to see the gauge. The Victor et al. gauge, while presumably somewhat easier to manipulate, must be precisely positioned or held to obtain accurate readings, and does not visually indicate to a mechanic or inspection officer whether the drum diameter passes or fails inspection.
Thus, there is a need for an apparatus for determining if the maximum allowable inside diameter of a brake drum has been exceeded, without requiring that the drum be removed from the wheel axle, that is simple and convenient to use, and in addition emits an audible and/or visual signal to an inspector when such maximum allowable diameter has been exceeded. The apparatus must be easy to insert and properly position or align on the braking surface of a brake drum when mounted on a vehicle axle, and should also be easy to operate and provide consistently reliable and accurate results. The present inventor, having personal experience with inspecting brake drums using known devices and recognizing the disadvantages thereof, has unexpectedly envisioned and subsequently designed a brake drum measuring device whereby rather than providing another mechanical gauge-type device that requires positioning between two or more diametrically opposite positions on the inside surface of the drum, the present inventor's apparatus need only be placed on the inside braking surface of the brake drum, and such apparatus will automatically calculate and signal whether or not the inside diameter has reached or exceeded the allowable maximum diameter.